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INSTRUCTIONS GEK— 45315In3ert Booklet GEK-4530/
TRANSFORMER DIFFERENTIAL RELAY
WITH PERCENTAGE AND HARMONIC RESTRAINT
TYPE STD28C
6 RESTRAINT CIRCUITS
GENERAL ELECTRIC
GEK-4531 5
TRANSFORMER DIFFERENTIAL RELAYWITH PERCENTAGE AND HARMONIC RESTRAINT
TYPE STD28C
INTRODUCTION
These instructions are a supplement to instruction book GEK-45307, which is attached. The combinationof the two form the instructions for the Type STO28C.
DESCRIPTION AND APPLICATION
The STD28C relay is a single phase harmonic restrained transformer percentage differential relay forthe protection of high voltage rectifier transformers. This relay is similar to the STDI5C, except thatthe harmonic restraint is provided by the second harmonic only so that the normal odd harmonic flowing ina rectifier transformer will not reduce the relay sensitivity. This harmonic selection is accomplished byprecisely tuned filters. However, the change in harmonic restraint does not change the application orsetting calculation as outlined for the STD15, except the STD28 relay is so designed that the harmonicrestraint may be set at 15 minimum.
Note that the STD28C relay have five (5) restraint windings as opposed to only three (3) as in theSTD51C.
Figure 1 illustrates the outline and panel drilling for the STD28C(-)D drawout case.
Figure 2 illustrates the internal connections diagram for the STD28C.
Figure 3 illustrates the external wiring diagram for the STD28C relay.
Figure 4 illustrates the test circuit for the STD relays. Note polarity when connecting DC sources.
TESTING INSTRUCTIONS
The STO28C relay may be tested per the instructions in the attached instruction book. In addition,since the harmonic restraint can be set at l5 by adjusting R2 such may be checked by adjusting the12(d-c) at 4.0 amps and the 11(a—c) (current into relay) at 8.1 amperes per the test circuit illustratedin Figure 4 with S2 switch closed to position A.
A tolerance of + 1. is acceptable, thus if the relay operated within 14 - 16 harmonic current restraint for the 15i calibration, no attempt should be made to obtain a more precise setting.
The following expression shows the relationship between the percent second harmonic, the d-ccomponent, and the b-pass current.
SECOND HARMONIC = .212 12 (d-c)X 100
.45 I (a—c) + 0.5 12 (d—c)
By setting the 12(d-c) at 4.0 amps, and solving for the second harmonic for 14 — 16’, thefollowing by—pass current levels are required:
C HR.; I (a-c)
14 9.115 8.116 7.3
This matter is discussed in more detail in the attached instruction book under the paragraphHarmonic Current Retraint.
These instructions Jo not purport to cover all details or variahons in equipment nor to provide for every possiblecontingency to be mi t in connection with installation, operation or maintenance. Should furlher information be desiredor should particular roblems arise which ore not covered sufficiently for the purchaser’s purposes, the mafler shouldbe referred to the G nerol Electric Company.
11 13 15 17 19u000000000
12 11, i6 1 20
3I. DR.20 HOlES
FIG. 1 (0178A7336—2) Outline And Panel Drilling For The Large, Double—Ended, Deep (L2D) Case Of The STD?8CRelay
GEK45315
N UMBER ThGOF STUDS
(FRONT VIEW)
PMEL LOCATION
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PANEL RILLING FORsx-iiui MTG (FRor viEw)
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ShORT FINGERRECTIHER LEAD
PsO. TO TEqH. BE).
Dl FFERENTI ALTNSF.
ThROUGH cuRENrRESTR. TRA4SF.
GEK—45315
FIG. 2 (0257A5034—O) Internal Connections Diagram For The STD28C Relay (Front View)
I’ 3 (E)c)
TiSt
TI‘I
20 (—Dc)
tEGEND
Dl FFERENT I ALCUR R £14 T
TO AP4YTAP
TNRU CUPREPsTIRAN SF0 R ‘• ?
- r rLiaj&
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2 67
5
GEK-45315
FIG. 3 (0165B2618-1) External Connections Diagram For The ST[?8 Relay
4
6
GEK-45315
(4)DC
2
-o
0
18 20
()13 1715
16
TEST PLUG XLAI2A
TEST CIRCUIT FOP STD2S RELAYS
FIG. 4 (0246A6970-1) Test Circuit For The STD28 Relay
GE Power Management
215 Anderson AvenueMarkham, OntarioCanada L6E 1B3Tel: (905) 294-6222Fax: (905) 201-2098www.ge.comlindsyslpm
GEK-45307K
INSTRUCTIONS
TRANSFORMER DIFFERENTIAL RELAYS WITH
PERCENTAGE AND HARMONIC RESTRAINT
TYPES STD15C and STD16C
GE Protection and Control205 Great Valley ParkwayMalvern, PA 19355-1337
GE K- 45307
DESCRIPTION 3APPLICATION 3CALCULATION OF SETTINGS 5
Method 5Current Transformer Connections 5Determination of CT Turns and Type STD Relay Tap Setting 5Current Transformer Ratio Error 6Percent Slope Setting 8I Determination of CT Turns and STD Relay Tap Setting 8II Percent Ratio Error 9IA Repeat - CT Turns and Relay Tap Setting 9hA Repeat - Percent Ratio Error 10III Percent Slope Setting 10
RATINGS 10Models 12STD15B and I2STDI6B 10Auxiliary Relay Control Circuit 11
CHARACTERISTICS iiPickup and Operating Time 11Overcurrent Unit Pickup 11Percentage Differential Characteristics 12Harmonic Restraint Characteristics 12
BURDENS 13CONSTRUCTION 14
Current Transformers 14Through—Current Restraint Circuit 14Differential—Current Circuit 15Overcurrent Unit 15Main Operating Unit 16Target and Seal—in Unit 16Case 16
RECEIVING, HANDLING AND STORAGE 17ACCEPTANCE TESTS 17
Visual Inspection 17Mechanical Inspection 17Electrical Tests 17Test Equipment 18
INSTALLATION PROCEDURE 18Tests 18Pickup 19Harmonic Current Restraint 19Through-Current Restraint 20Instantaneous Overcurrent Unit 21Dropout of Main Unit 21Location 21Mounting 21Connections 21
ADJUSTMENTS 21Tap Plug Positioning 21Percent Slope Setting 23
OPERATION 23Targets 23Disabling of Type STD Relay 23
MAINTENANCE 23Contact Cleaning 23
PERIODIC TESTS 24Pickup 24Harmonic Current Restraint 24Through-Current Restraint 25
SERVICING 25RENEWAL PARTS 26
2
GEK-45 307
TRANSFORMER DIFFERENTIAL RELAYSWITH PERCENTAGE AND HARMONIC RESTRAINT
STD15C and STD16C
INTRODUCTION
Relays of the STD type are transformer differential relays provided with thefeatures of percentage and harmonic restraint. A static decision unit controls asmall telephone-type relay that provides the contact output.
Percentage restraint permits accurate discrimination between internal and externalfaults at high current, while harmonic restraint enables the relay to distinguish,by the difference in waveform, between the differential current caused by aninternal fault, and that of transformer magnetizing inrush.
DESCRIPTION
Each Type STD relay is a single-phase unit. The Type STD15C relay is designed to beused for the protection of two-winding power transformers and has two through-current restraint circuits and one differential—current circuit.
The Type STU16C relay is designed for use with three-winding power transformers andhas three through—current restraint circuits and one differential—current circuit.It may also be used for four-circuit transformer protection (see Figure 1) when onlythree circuits require through-current restraint, while the fourth circuit, beingthe weakest, needs no through-current restraint.
APPLICATION
The current transformer ratios and relay taps should be selected to obtain themaximum sensitivity without risking thermal overload of the relay or currenttransformer (CT), or the possibility of misoperation. Therefore, currenttransformer ratios in the various windings of the power transformer should beselected with the following points in mind:
1. The lower the relay tap and the lower the CT ratio selected, the higher will bethe sensitivity. However, the lowest CT ratio and the lowest relay tap may nothe compatible with some of the following restrictions. Where a choice isavailable of increasing either the CT ratio or the relay tap, it is desirableto increase the CT ratio in preference to the relay tap.
Since the relay burden is likely to be small compared to the lead burden,increasing the CT ratio tends to improve the relative performance of the CTs,as a result of reducing the maximum secondary fault current and increasing theaccuracy of the CTs.
These Instructions do not purport to cover all details or Variations lfl equipment nor to provide forevery possible cxtingency to be met in connection with installation, operation or Saantnce. Shouldfurther InforMation be desired or should particular probleirm arise which are not covered wfficiently forthe purchasers purposes, the iIsltter should be referred to the General Liectric COmg)auIy.
To the extent required the products described herein meet applicable ANSI, 1555 and standards;but no such assurance is g;ven with respect to local codes and ordinances because they varq greatly.
3
GE K—45 307
2. The CT secondary current should not exceed the continuous thermal rating of theCT secondary winding.
3. The relay current corresponding to maximum kVA (on a forced—cooled basis)should not exceed twice tap value, which is the thermal rating of the relay.
4. The CT ratios should be high enough that the secondary currents will not damagethe relay under maximum internal fault conditions (refer to RATINGS).
5. The relay current corresponding to rated kVA of the power transformer (on self-cooled basis) should not exceed the relay tap value selected (magnetizinginrush night operate the instantaneous overcurrent unit). If the transformerunder consideration does not have a self—cooled rating, the transformernianufacturer should be consulted for the “equivalent self-cooled rating”; thatis the rating of a self-cooled transformer that would have the same magnetizinginrush characteristics as the transformer being considered.
6. The current transformer tap chosen must be able to supply the relay with 8times rated relay tap current, with an error of less than 20% of the totalcurrent. If the current transformers produce an error of greater than 20% atless than 8 times tap value, the harmonic content of the secondary current maybe sufficient to cause false restraint on internal faults.
7. The CT ratios should be selected to provide balanced secondary current onexternal faults. Since it is rarely possible to match the secondary currentsexactly by selection of current transformer ratios, ratio-matching taps areprovided on the relay by means of which the currents may usually be matchedwithin 5%. When the protected transformer is equipped with load-ratio controlit is obvious that a close match cannot be obtained at all points of the ratio-changing range. In this case, the secondary currents are matched at the middleof the range and the percentage_differential characteristic of the relay isrelied upon to prevent relay operation on the unbalanced current which flowswhen the load-ratio control is at the ends of the range.
8. In some applications, the power transformer will be connected to the highvoltage or low voltage system through four breakers (as shown in Figure 1) asfor example in a ring-bus arrangement. In this case, the CT ratios must beselected so that the secondary windings will not be thermally overloaded onload current flowing around the ring in addition to the transformer loadcurrent. It is recommended that CTs on each of the two low voltage (or highvoltage) breakers be connected to a separate restraining winding to assurerestraint on heavy through-fault current flowing around the ring bus.
It is not desirable to protect two parallel transformer banks with one set ofdifferential protection, since the sensitivity of the protection would be reduced.In addition, if the banks can be switched separately, there is a possibility offalse operation on magnetizing inrush to one transformer bank, causing a“sympathetic inrush” into the bank already energized. In this case, the harmonicstend to flow between the banks, with the possibility that there will be insufficientharmonics in the relay current to restrain the relay.
Typical elementary diagrams for the STD15C and STD16C are illustrated in Figures 2and 3.
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GEK—45307
CALCULATION OF SETTINGS
METHOD
The calculations required for determining the proper relay and CT taps are outlinedbelow. A sample calculation, for the transformer shown in Figure 4, is then given.
CURRENT TRANSFORMER CONNECTIONS
Power Transformer Connections Current Transformer Connections
Delta-Wye Wye-DeltaWye-Delta Oelta-WyeDelta-Delta Wye-WyeWye-Wye Delta-DeltaDelta-Zigzag with 00 phase Delta—Deltashift between primary andsecondary
DETERMINATION OF CT TURNS AND TYPE STD RELAY TAP SETTING
1. Determine the maximum line currents (Max. I) on the basis that each powertransformer winding may carry the maximum forced-cooled rated kVA of thetransformer.
— Maximum Transformer kVAMax. P - (Line kV)
2. Determine the full-load rated line currents (100% I) on the basis that eachpower transformer winding may carry the full self-cooled rated kVA of thetransformer, or the equivalentli self-cooled ratings.
- 100% Transformer kVA100% ‘P
— ,j (Line kV)
Actually, this calculation does not mean that all windings will necessarilycarry these maximum load currents continuously. This is only a convenient wayof calculating the currents in the other windings in proportion to theirvoltage ratings. This is the requirement for selecting the relay tap settingso that the relay will not operate for any external fault.
3. Select CT ratios so that the secondary current corresponding to maximum Ip doesnot exceed the CT secondary thermal rating (5 amperes). In the case where atransformer is connected to a ring bus, for example, the CT ratio should beselected so that the CT thermal rating will not be exceeded by the maximum loadcurrent in either breaker. Also, select CT ratios so that the relay currentscan be properly matched by means of the relay taps. (Highest current not morethan 3 times lowest current).
For Wye-connected CTs
Tap Current 100% ‘P
5
GEK-45307
For Delta-connected CTs
Tap Current 100%N
where N is the number of CT secondary turns.
4. Check the matching of relay currents to relay taps, to keep the mismatch erroras low as possible.
Calculate the percentage of mismatch as follows: on two-winding transformers,determine the ratio of the two relay currents and the tap values selected. Thedifferences between these ratios, divided by the smaller ratio, is the percentof mismatch. The mismatch normally should not exceed 5%.
For three-winding transformers, the percent of mismatch error should be checkedfor all combinations of currents or taps.
If taps cannot be selected to keep this percentage error within allowablelimits, it will be necessary to choose a different CT ratio on one or morelines, to obtain a better match between relay currents and relay taps.
5. Check to see that the sum of the relay currents that will be applied to therelay for a fault at the terminals of the power transformer is less than 220amperes RMS for 1 second. If the period during which a fault current flows inthe relay can be definitely limited to a shorter time, a higher current can beaccommodated in accordance with the relation:
(Amperes)2 x seconds 48,400
Also check that the sum of the multiples of tap current on an internal orexternal fault do not exceed 150.
CURRENT TRANSFORMER RATIO ERROR
The CT ratio error must be less than 20% at 8 times relay rated tap current. Thisis based on the instantaneous unit being set at its normal setting, which is 8 timestap rating. If the instantaneous unit pickup is raised above this value, the 20%figure must be reduced, as described in the CHARACTERISTICS section.
The calculations listed below are for the worst—fault condition, as far as CTperformance is concerned, which is an internal ground fault between the CT and thetransformer winding, with none of the fault current supplied through the neutral ofthe protected transformer.
1. Determine the burden on each CT, using the following expressions:
a. For Wye-connected CTs
Z = B + Ne+2f+ 2R Ohms (Equation 1)
b. For Delta-connected CTs
Z = 2B + Ne + 2f+ 2R Ohms (Equation 2)
1000
6
GEK—45307
where B STD relay total burden (see Table I)
N = number of turns in bushing CT
e = bushing CT resistance per turn, milliohms (at maximum expectedtemperature)
f busing CT resistance per lead, milliohms (at maximum expectedtemperature)
R one-way control cable lead resistance (at maximum expectedtempera ture)
TABLE I
Total Burden for 60 Cycle Relays
STD TAPS AMPS 8 X TAP AMPS BURDEN OHMS (B) MIN P.U. AMPS2.9 23.2 0.180 0.873.2 25.6 0.156 0.963.5 28.0 0.140 1.053.8 30.4 0.120 1.144.2 33.6 0.112 1.264.6 36.8 0.096 1.385.0 40.0 0.088 1.508.7 69.6 0.048 2.61
2. Determine CT secondary current for 8 times tap setting.
= 8 x STD relay tap rating
(Note: For the location of fault assumed, all the faultcurrent is supplied by one CT, so that CT current andrelay current are the same, regardless of whether theCTs are connected in wye or delta.)
3. Determine secondary CT voltage required at 8 times tap setting.
Esec = I5Z
4. From excitation curve of particular tap of current transformer being used,determine excitation current ‘C. corresponding to this secondary voltage,Esec.
5. Determine the percent error in each CT by the expression:
% error =‘C X 100‘S
This should not exceed 20% of any set of CTs. If it does, it will benecessary to choose a higher tap on that set of CTs, and repeat thecalculations on selection of relay taps, mismatch error, and percent ratioerror.
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GEK-45307
PERCENT SLOPE SETTING
The proper percent slope required is determined by the sum of:
a. The maximum range of manual taps and the load—ratio-control, or automatictap changing means, in percent.
b. The max iinuni percent of mismatch of the relay taps.
Set the desired percent slope by means of R3 (See Figure 6A).
The percentage slope setting selected should be greater than the ratio of maximumtotal error current to the smaller of the through currents. In general, if thetotal error current does not exceed 20%, the 25% setting is used. If it exceeds20%, but not 35%, the 40% setting is used.
If the movable lead is used (as in Figure 1, for example) the percent slope settingshould be chosen about twice as high, since the movable lead provides no restraint.
EXAMPLE (REFER TO FIGURE 4)
I. Determination of CT Turns and STD Relay Tap Settings
1. Transformer and Line A B C2. Maximum I = 3750/ /3 (Line kV) 19.7 49.5 1573. 100% 1p = 3000/ /! (Line kV) 15.7 39.6 1254. Assume CT turns (N) 20 20 605. Maximum I secondary (less than 5a) 0.98 2.47 2.626. 100% I secondary 0.79 1.98 2.087. CT connections Delta Wye Delta8. Relay Current for 100% I Sec. 1.37 1.98 3.60
Select a relay tap for one of the line currents and calculate what thecurrents in other lines would be if they were increased in the same ratio.If any current is greater than V’3 times any other, the 8.7 tap should bechosen for it, and new ideal relay taps calculated for the other lines.
9. Ideal Relay Taps (Set C = 8.7) 3.31 4.78 8.710. Try Relay Taps 3.2 4.6 8.711. Check Mismatch Error
Ratio of Taps on Lines B-A -4 = 1.43 Ratio of Sec. Lines Currents
1.98 -
1.37 — 1.44
• 1.44 - 1.43Mismatch1 43 = 0.7%
Ratio of Taps on LinesC-B46
1.89 Ratio of Sec. Line Currents = 1.82
• 1.89 - 1.82Mismatch1 82 = 3.8%
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GEK—45307
Ratio of Taps on Lines C-A = 2.72 Ratio of Sec.Line Currents 2.63
Mismatch 2.72 -2.63=
(All are less than 5%; therefore, mismatch error is not excessive)
12. Check that the sum of the maximum relay currents is less than 220 amps for1 second, and therefore, short-time rating of relay is not exceeded.
II. Percent Ratio Error
expected temperatures)(R) = 0.284 ohms(e) = 4 milliohms(e) = 2.5(e) = 2.3(f) = 75 milliohms(f) = 52.5(f) = 18.6
a. Line A, Z = 2 (0.156) + (20x4) + (2.0 x 75)1000
= 0.312 + 0.205 + 0.568 = 1.085
A1.085
25.627.8
1 .003.4%
Exciting current on line B is too high; shouldimprove CT performance.
IA - Repeat - CT Turns and Relay Tap Setting
+ 2 (.284)
B0.8
36.829.450
136%
C0.833
69.658.00.50.8%
try higher tap on CT to
1. 100% I2. Try CT turns (necessary to change C
also for proper matching)3. 100% I secondary4. Relay Current5. Ideal Relay Taps (Set C 8.7)6. Use Relay Taps7. Mismatch Error is less than 5%
15.7 39.6 125
20 40 800.79 0.991.37 0.994.40 3.194.6 3.2
ASSUME (all measured at their maximumOne-way CONTROL CABLE RESISTANCEBushing A CT resistance per turn
BII
CBushing A CT resistance per lead
BII II
C
1. Burdens on CTs, using Equation 1 or Equation 2 from page 6.
b. Line B, Z = 0.096+ (20 x 2.5) + (2.0 x 52.5)
+ 0.5681000
= 0.096 + 0.138 + 0.568 0.80
c. Line C, Z 2 (0.048) + (60 x 2.3) + (2 x 18.6)1000
= 0.096 + 0.180 + 0.568 = 0.833
+ 0.568
2. Impedance, ohms3. 8 times tap, amperes4. ES CT voltage require (IZ)5. I required, from excitation curve6. % Ratio Error
1 .562.708.78.7
9
GEK-45307
hA Repeat - Percent Ratio Error
1. Burden on CTs
Line A, Z = 0.192 + 0.205 + 0.568 = 0.965Line B, Z = 0.156 + 0.188 + 0.568 = 0.912Line C, 7 = 0.096 + 0.226 + 0.568 = 0.890
2. Impedance Ohms 0.965 0.912 0.8903. 8 times Tap, Amperes 36.8 25.6 69.64. CT voltage required (IZ) 35.6 23.4 61.9• TE required, from excitation curve 1.1 0.25 0.176. % of Ratio Error 3.1% 1.0% 0.3%
Percent error is less than 20%, so CT taps and relay taps are satisfactory.
III Percent Slope Setting
1. Assume load ratio control maximum range 10.0%2. Relay tap mismatch, from IA above (Lines A-B) 4.6%
Use 25% setting 14.6%
RATI NGSMODELS 12STD15C AND 12STD16C
Continuous Rating
The through—current transformer and differential-current transformer will standtwice tap value for any combination of taps, or they will stand twice tap value ifall but one of the restraint windings carry 0 current, and the full restraintcurrent (equal to twice tap value) flows through the differential—currenttrans former.
Short Time Rating (Thermal)
220 amperes for 1 second, measured in the primary of any transformer of the type STDrelay. Higher currents may be applied for shorter lengths of time in accordancewith the following equation:
12t 48,400
where I = current amperest time in seconds.
Short Time (Electrical
For both the STD15C and STD16C the sum of the multiples of tap current fed to therelay from the several sets of current transformers should not exceed 150. Thesemultiples should be calculated on the basis of RMS symmetrical fault current. Thislimitation is a result of the voltage rating of the rectifiers in the through—current restraint circuit. Note that in Figure 1 external fault current can flowthrough circuit breakers 52-1 and 52-2 without being limited by the transformerimpedance.
10
GE K—45 307
TABLE II
TARGET AND SEAL-IN UNIT2.0 Amp Tap 0.6 Amp Tap 0.2 Amp Tap
DC Resistance 0.13 Ohms 0.6 Ohms 7 ohmsCarry Continuously 0.5 Amps 1.5 Amps 0.25 AmpsCarry 30 Amps for 4 Secs. 0.5 Secs.Carry 10 Amps for 30 Secs 4 Secs. 0.2 Secs.
AUXILIARY RELAY CONTROL CIRCUIT
The STD15C and STD16C relays are available for use with 48, 125, and 250 DC or 48,110 and 220 DC control voltage, depending upon the relay model. A plate with smalllinks located on the front of the relay enables the selection of one of thesevoltages.
The STD relay is provided with two open contacts connected to a common outputcircuit. The current—closing rating of the contact is 30 amps for voltages notexceeding 250 volts. If more than one circuit breaker is to be tripped, or if thetripping current exceeds 30 amperes, an auxiliary relay must be used with the STOrelay. After the breaker trips, it is necessary that the tripping circuit of theserelays (STD and auxiliary) be de—energized by an auxiliary switch on the circuitbreaker or by other automatic provisions. A manual reset relay is recommended andnormally used.
CHARACTERISTI CS
PICKUP AND OPERATING TIME
The operating characteristic is shown in Figure 7. The curve for various percentageslopes shows the percent slope versus the throughcurrent flowing in the transformer.The percentage slope is a figure given to a particular slope tap setting, andindicates an approximate slope characteristic. Pickup at zero restraint isapproximately 30% of tap value (see Table III). The dropout time, when theoperating current is reduced to zero from any value above pickup, is less than 25mi lii seconds.
Curves of the operating time of the main unit and of the instantaneous unit areshown in Figure 5, plotted against differential current. The main unit operatingtime includes auxiliary unit operating time.
OVERCURRENT UNIT PICKUP
The overcurrent unit is adjusted to pick up when the differential currenttransformer ampere-turns are 8 times the ampere turns produced by rated tap currentflowing in that tap. For example:
When only one CT supplies current, and the tap plug for the CT is in the 5ampere tap, 40 amperes are required for pickup. This pickup value is based onthe AC component of current transformer output only, since the differentialcurrent transformer in the relay produces only a half cycle of any DC (offset)component present.
11
GEK—45307
If ratio matching taps are chosen so that rated CT current is not greater than thetap rating on a self-cooled basis, the overcurrent unit will not pick up onmagnetizing inrush. If CT currents are greater than tap rating, there is dangerthat the unit may pick up, especially on small transformer banks. If this happens,it is recommended that the CT ratio or relay tap setting be increased, rather thanincreasing the pickup of the overcurrent unit. If the overcurrent setting must beraised, the requirements on CT error will be more stringent, in accordance with thefollowing equation:
E 20 - (2.5) (P-8)
where E = CT error current in percent, at pickup of the overcurrent unitP = Pickup of overcurrent in multiples of tap setting.
PERCENTAGE DIFFERENTIAL CHARACTERISTICS
The percentage differential characteristics are provided by through—currentrestraint circuits. In addition to the operating circuit, which is energized by thedifferential current of the line current transformers, the relay is equipped with arestraining circuit that is indirectly energized by the transformer secondarycurrents. For the relay to operate, the current transformer secondary currents mustbe unbalanced by a certain minimum percentage, determined by the relay slope setting(as shown in Figure 7). This characteristic is necessary to prevent false operationon through-fault currents. High currents saturate the cores of the currenttransformers and cause their ratios to change, with the result that the secondarycurrents become unbalanced. Percentage restraint is also required to preventoperation by the unbalanced currents caused by imperfect matching of the secondarycurrents, as previously described under Determination of CT Turns and STD Relay TapSettings.
HARMONIC RESTRAINT CHARACTERISTICS
At the time a power transformer is energized, current is supplied to the primarythat establishes the required flux in the core. This current is called magnetizinginrush, and in the primary winding flows only through the current transformers.This causes an unbalance current to flow in the differential relay, which wouldcause false operation if means were not provided to prevent it.
Power system fault currents are of a nearly pure sine waveform, plus a DC transientcomponent. The sine waveform results from sinusoidal voltage generation and nearlyconstant circuit impedance. The DC component depends on the time in the voltagecycle at which fault occurs, and upon circuit impedance magnitude and angle.
Transformer magnetizing—inrush currents vary according to the extremely variableexciting impedance resulting from core saturation. They are often of highmagnitude, occasionally having an RMS value with 100% offset, approaching 16 timesfull-load current for worst conditions of power transformer residual flux and pointof-circuit closure on the voltage wave. They have a very distorted waveform made upof sharply peaked half-cycle ioops of current on one side of the zero axis, andpractically no current during the opposite half cycles. The two current waves areillustrated in Figure 8.
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GEK—45307
Any current of distorted, nonsinusoidal waveform may be considered as being composedof a DC component plus a number of sine-wave components of different frequencies;one of the fundamental system frequency, and the others, called “harmonics, havingfrequencies which are 2, 3, 4, 5, etc., times the fundamental frequency. Therelative magnitudes and phase positions of the harmonics with reference to thefundamental determine the waveform. When analyzed in this manner, the typicalfault-current wave is found to contain only a very small percentage of harmonics,while the typical magnetizing-inrush—current wave contains a considerable amount.
The high percentage of harmonic currents in the magnetizing-inrush—current waveafford an excellent means of distinguishing it electrically from the fault-currentwave. In the Type STD relays, the harmonic components are separated from thefundamental component by suitable electric filters. The harmonic current componentsare passed through the restraining circuit of the relay, while the fundamentalcomponent is passed through the operating circuit. The DC component present in boththe magnetizing-inrush— and offset-fault—current waves is largely blocked by theauxiliary differential-current transformer inside the relay, and produces only aslight momentary restraining effect. Relay operation occurs on differential—currentwaves in which the ratio of harmonics to fundamental is lower than a givenpredetermined value, for which the relay is set (e.g. an internal fault—currentwave), and is restrained on differential—current waves in which the ratio exceedsthis value (e.g. magnetizing-inrush—current wave).
BURDENS
Burdens are shown in Table III and IV. Burdens and minimum pickup values aresubstantially independent of the percent slope settings, and are all approximately100% power factor. Figures given are burdens imposed on each current transformer at5.0 amperes.
TABLE III
ZERO OPERATING CIRCUIT * RESTRAINT CIRCUITTARESTRAINT 60 CYCLE RELAYS 60 CYCLE RELAYSET11.
PICKUP BURDEN IMPEDANCE BURDEN IMPEDANCEMPSAMPS VA OHMS VA OHMS
2.9 0.87 3.2 0.128 1.3 0.0523.2 0.96 2.7 0.108 1.2 0.0483.5 1.05 2.4 0.096 1.1 0.0443.8 1.14 2.0 0.080 1.0 0.0404.2 1.26 1.9 0.076 0.9 0.0364.6 1.38 1.6 0.064 0.8 0.0325.0 1.50 1.5 0.060 0.7 0.0288.7 2.61 0.7 0.028 0.5 0.020* Burden of operating coil is 0 under normal conditions.Burden of 50-cycle relay is the same or slightly lower.
TABLE IV
f DC CONTROL CIRCUIT BURDENRATED VOLTS 48 125 250 48 110 220MILLIAMPS 140 105 88 140 96 80
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GEK-45307
CONSTRUCTION
Figure 6 shows the internal arrangement of the components of the STD15C relay.
Refer also to the internal connection diagrams, Figures 10 and 11, which will
identify the parts more completely.
CURRENT TRANSFORMERS
In the Type STD15C relay, the through-current transformer has two primary windings,
one for each line-current-transformer circuit. Winding No. 1 terminates at stud 6
and winding No. 2 terminates at stud 4.
In the Type STD16C relay, there are three separate through-current transformers,
each with only one primary winding, and each terminating at a separate stud,
windings No. 1, No. 2 and No. 3 corresponding to studs 6, 4 and 3 in that order.
In both relays there is a differential-current transformer with one primary lead
brought out to stud 5.
The primary circuit of each of these transformers is completed through a special tap
block arrangement. Two or three horizontal rows of tap positions are provided
(depending on whether the relay is a Type STD15C or STO16C), one row for each
through-current transformer winding. A tap on the differential—current transformer
is connected to a corresponding tap of the through-current restraint windings by
inserting tap plugs in the tap blocks.
When the STD16C relay is used on four—circuit applications, as shown in Figure 1,
the fourth circuit CT is connected to stud 7, and the jumper normally connected
between terminals 6 and 7 at the rear of the relay cradle should be disconnected at
the terminal 6 end and reconnected to the upper row in the tap block (above the row
marked winding 1), which connects it directly to the differential-current
transformer in the STD relay. The terminal on the movable lead should be placed
under the tap screw that gives the best current match for the current in the movable
lead.
The taps permit matching of unequal line-current transformer secondary currents.
The tap connections are so arranged that in matching the secondary currents, when a
tap plug is moved from one position to another in a horizontal row, corresponding
taps on both the differential—current transformer winding and one of the through—
current transformer windings are simultaneously selected so that the percent
through-current restraint remains constant.
It should be recognized that pickup current flows not only through differential-
current transformer but also through one of the primary windings of the through-
current transformer, producing some restraint. However, compared to the operating
energy, this quantity of restraint is so small that it may be assumed to be zero.
THROUGH-CURRENT RESTRAINT CIRCUIT
A full wave bridge rectifier receives the output of the secondary of each through
current restraint transformer. In the STD16C relay, the DC outputs of all three
units are connected in parallel. The total output is directed to the percent slope
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GEK—45307
rheostat (R3) located on the front of the relay. By means of adjusting therheostat, the percent slope may be varied from 15% to 40%. The output is putthrough an isolating transformer, rectified, and directed to the sensitive solidstate amplifier that controls the telephone—type relay.
DIFFERENTIAL-CURRENT CIRCUIT
The differential-current transformer secondary supplies 1) the instantaneous unitdirectly; 2) the operating (tripping) signal to the solid-state amplifier through aseries-tuned circuit; and 3) the harmonic restraint isolating transformer through aparallel resonant filter. The operating and restraint currents are each rectifiedby a full—wave bridge prior to being supplied to the sensitive sense amplifier.
The series resonant circuit is made up of a 5 rnicrofarad capacitor (Cl) and areactor (Li) that are tuned to pass currents of the fundamental system frequency andto offer high impedance to currents of other frequencies. Resistor Ri is connectedin parallel on the AC side of the operate rectifier, and can be adjusted to give thedesired amount of operate current. The output of the rectifier is applied to theoperating circuit of the sense amplifier.
The parallel resonant trap is made of a 15 microfarad capacitor (C2) and areactor (L2) that are tuned to block fundamental frequency currents while allowingcurrents of harmonic frequencies to pass with relatively little impedance. ResistorR2 is connected in parallel on the AC side of the harmonic restraint rectifier, andcan be adjusted to give the desired amount of harmonic restraint. The output of therectifier is paralleled with the through-current restraint currents and applied tothe restraint circuit of the sense amplifier.
It will be evident that if the differential current applied to the relay issinusoidal and of system frequency, it will flow mostly in the operating circuit andhence cause the relay to yield an output. If, however, the differential circuitcontains more than a certain percentage of harmonics, the relay will be restrainedfrom operating by the harmonic currents flowing in the restraint circuit.
A IhyriteR resistor connected across the secondary of the differential—currenttransformer limits any momentary high-voltage peaks which may occur, thus protectingthe rectifiers and capacitors from damage, without materially affecting thecharacteristics of the relay.
OVERCURRENT UNIT
The instantaneous unit is a hinged-armature relay with a self—contained targetindicator. On extremely heavy internal fault currents, this unit will pick up andcomplete the trip circuit. The instantaneous unit target will be exposed, toindicated that tripping was through the instantaneous unit.
Because of saturation of the CTs and relay transformers at high fault currents, itis possible that less operating currents will be provided from the differential—current transformer than the percentage slope tap would imply, and more harmonicrestraint will be provided than the actual harmonic content of the fault currentwould supply. As a result, under conditions of a high internal fault current, themain unit may be falsely restrained. Tripping is assured, however, by the
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GEK—45307
overcurrent unit operation. Pickup is set above the level of differential currentproduced by maximum magnetizing inrush current. Figure 5 shows the relative levelsof pickup and speed of operation of the main unit and the overcurrent unit.
MAIN OPERATING UNIT
The primary functioning unit of the STD relay is a solid-state amplifier, whoseoutput controls a simple telephone relay. The sense amplifier is shown in Figures10 and 11 as a large rectangle. The amplifier consists of many electroniccomponents mounted on a printed circuit card in the top half of the relay. Thisprinted circuit card is installed in a ten-prong printed card design socket. Aschematic of this card is shown in Figure 9. This component is adjusted prior toleaving the factory, and should require no further attention.
The telephone-type relay is mounted vertically in the mid-section of the relay. It,too, has been carefully adjusted at the factory, and should require no furtherattention. If this small relay has been disturbed, refer to the section underADJUSTMENTS.
TARGET AND SEAL-IN UNIT
There is a target and seal-in unit mounted on the top left of the relay. This unithas its coil in series and its contacts in parallel with the main contacts of thetelephone-type relay. When the telephone-type relay contacts close, the seal-inunit operates, raising its target into view and sealing around the telephone-typecontacts. The target of this unit will remain exposed until released by pushing abutton beneath the lower left corner of the cover of the relay case.
CASE
The case is suitable for surface or semi—flush panel mounting, and an assortment ofhardware is provided for either method. The cover attaches to the case, and carriesthe target reset mechanism for the trip indicator and instantaneous unit. Eachcover screw has provision for a sealing wire.
The case has studs or screw connections at the bottom for the external connections.The electrical connections between the relay unit and the case studs are madethrough spring-backed contact fingers mounted in stationary molded inner and outerblocks, between which rests a removable connecting plug that completes the circuit.The outer block, attached to the case, holds the studs for the external connections,and the inner block has terminals for the internal connections.
The relay mechanism is mounted in a steel framework called the cradle, and is acomplete unit, with all leads terminating at the inner block. This cradle is heldfirmly in the case by a latch at the top and bottom and a guide pin at the back ofthe case. The case and cradle are so constructed that the relay cannot be insertedin the case upside down. The connecting plug, besides making the electricalconnection between the blocks of the cradle and case, also locks the latch in place.The cover, which is fastened to the case by thurnbscrews, holds the connecting plugin place.
To draw out the relay unit, the cover is removed and the plug is drawn out.Shorting bars are provided in the case to short the current-transformer circuits
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GEK-45307
(see Figure 12). The latches are then released and the relay unit can be easilydrawn out.
A separate testing plug can be inserted in place of the connecting plug to test therelay in place on the panel, either from its own source of current, or from othersources. Or, the relay unit can be drawn out and replaced by another which has beentested in the laboratory.
RECEIVING. HANDLING AND STORAGE
These relays, when not included as part of a control panel, will be shipped incartons designed to protect them against damage. Immediately upon receipt of arelay, examine it for any damage sustained in transit. If injury or damageresulting from rough handling is evident, file a damage claim at once with thetransportation company and promptly notify the nearest General Electric SalesOffice.
Reasonable care should be exercised in unpacking the relay, in order that none ofthe parts may be injured or the adjustments disturbed.
If the relays are not to be installed immediately, they should be stored in theiroriginal cartons in a place that is free from moisture, dust and metallic chips.Foreign matter collected on the outside of the case may find its way inside when thecover is removed, and cause trouble in the operation of the relay.
ACCEPTANCE TESTS
Immediately upon receipt of the relay, an inspection and acceptance test should bemade to make sure that the relay has not been damaged in shipment and that the relaycalibrations are unchanged.
VISUAL INSPECTION
Check the nameplate stamping to make sure that the model number, rating andcalibration range of the relay received agree with the requisition.
Remove the relay from its case and check by visual inspection that there are nobroken or cracked molded parts, or other signs of physical damage, and that allscrews are tight.
MECHANICAL INSPECTION
Check the operation of the telephone—type relay and instantaneous overcurrent unitmanually, to see that they operate smoothly, without noticeable friction or binds.Check the contact gap and wipe of these units, which should agree with values givenin the section on SERVICING.
ELECTRICAL TESTS
It is recommended that the following electrical tests be made immediately uponreceipt of the relay.
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GEK-45307
1. Minimum pickup of main operating unit.2. Minimum pickup of the instantaneous overcurrent unit.3. A single check point on the harmonic restraint characteristic.4. A single check point on the slope characteristic curve, for the approximateslope expected to be used.
Test Equipment
In order to facilitate tests, the following test equipment is recommended:
1. Two load boxes for regulating the test currents2. Three ammeters (two AC and one DC) for measuring the test currents3. A test rectifier for checking the relay’s response to the second harmonic. (SeeFigure 13.)4. One indicating lamp5. Two single-pole double—throw switch selectors6. A double—pole single-throw line switch.
Check the pickup of the main unit, using the connections shown in Figure 14.During this test, the selector switches (S2 and S4) are open, and current passesthrough one restraint winding and the operate winding only. For example, with arelay set with a 25% slope and at the 2.9 ampere ratio-matching taps, the main unitshould pick up at 30% of tap rating, 1O%, or the pickup should be between 0.78 and0.96 amperes. A source of DC power at rated voltage should be connected as shown inFigure 14; the indicating lamp will provide a signal, showing that the main unithas operated.
For an additional pickup test, the pickup should be 1.5 amperes, with currentflowing in terminals 5 and 6 and the tap plugs in the 5 ampere tap and the 25% slopetap position. If the pickup is between 1.35 and 1.65 amperes, no adjustment shouldbe made. The pickup of the relay has wider permissible variations than mostprotective relays, but due to the relay design and application, the relay accuracyis entirely adequate under all conditions, even during transformer magnetizinginrush or severe fault conditions.
With the selector switch, ST, in the A position, check the harmonic currentrestraint, as described in the section on INSTALlATION PROCEDURE.
The instantaneous overcurrent unit should be checked by passing a high currentthrough the 5—6 terminals. The pickup should be about eight times tap rating.
Check through-current restraint, as described in the section on INSTALLATIONPROCEDURE.
INSTALLATION PROCEDURE
TESTS
Before placing the relays in service, check the relay calibration to be used in itsfinal location, to ensure that it is correct. The following test procedure isoutlined for this purpose.
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GEK-45307
CAUTION
j The Relay calibration is accomplished by adjusting resistors Ri, R2 andR3. Changes made in any one of these resistors will affect the other twosettings. In the event one setting is changed, the pickup, harmonicrestraint, and through—current restraint adjustment procedures should berepeated until no further deviation from proper calibration is noted.Best results can be obtained if the through-current restraint adjustmentis made only after the other two settings are correct.
PICKUP
The test circuit for pickup is shown in Figure 14, with S2 open. The first valuesgiven are for a 5 ampere rated relay. Those in C ) are for a 1 ampere rated relay.Pickup should be 1.5 (0.3) amperes with current flowing in terminals 5 and 6, andthe tap plugs in the 5 (1.0) ampere position, and 25% slope tap setting. The pickupoperation should be repeated several times, until two successive readings agreewithin 0.01 ampere with the total pickup current being interrupted betweensuccessive checks. If pickup is found to be from 1.35 to 1.65 (0.27 to 0.33), thesetting should not be disturbed.
With DC control voltage applied to the proper terminals of the relay, the pickup ofthe telephone-type relay can be used as an indication of operation of the amplifier.This voltage may be applied as shown in Figure 15, and the indicating lamp willverify that the amplifier has produced an output signal.
Before Ri is adjusted for pickup, put a DC voltmeter (one volt scale) on pin 2 (-)and pin 8 (+) of the sense amplifier card. The pins are counted from right to leftwhen viewed from the card socket wiring connections. Apply pickup current andadjust Ri for a voltage input to the sense amplifier card of 0.430 to 0.470 volts.If the pickup is not in limits with this setting, then adjust P1 on the senseamplifier card to obtain the proper pickup value.
HARMONIC CURRENT RESTRAINT
The harmonic restraint is adjusted by means of a Test Rectifier used in conjunctionwith suitable ammeters and load boxes. The test circuit is as shown in Figure 15,with S2 closed to position A. Tests should be made on the 5.0 (1.0) ampere and 25%slope taps.
The analysis of a single-phase half-wave rectified current shows the presence offixed percentages of DC, fundamental, and second harmonic components, as well asnegligible percentages of all higher even harmonics. This closely approximates atypical transformer inrush current, as seen at the relay terminals, inasmuch as itsprincipal components are DC, fundamental, and second harmonic. Although the percentsecond harmonic is fixed, the overall percentage may be varied by providing a pathfor a controlled amount of by-passed current of fundamental frequency. The bypassed current is added in phase with the fundamental component of the half-waverectified current, and thus provides a means of varying the ratio of second harmoniccurrent to fundamental current.
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GEK-45307
The following expression shows the relationship between the percent second harmonic,the DC component, and the by-pass current.
% Second Harmonic = 0.212 ‘DC , 100
O.45Ii + 0.5IDCFigure 16 is derived from the above expression. It shows the percent secondharmonic corresponding to various values of by-pass current (Ii) for a constant DCset at 4.0 (0.2) amperes.
The relay is calibrated with a composite RMS current of two times tap value. Whenproperly set, the relay will restrain with greater than 20% second harmonic, butwill operate with second harmonic equal to 20% or lower. With the DC ammeter (12)set at 4.0 (0.8) amperes, the auxiliary relay should just begin to close itscontacts with gradually increasing bypass current (‘i) at a value of 4.5-5.5 (0.9-1.1) amperes. This corresponds to 19-21% second harmonic (see Figure 16), providinga 2 tolerance at the set point to compensate for normal fluctuations in pickup. Itshould be noted that the current magnitude in the rectifier branch (12) is slightlyinfluenced by the application of by-pass current (Ii) and should be checked to makesure it is maintained at its proper value.
If harmonic restraint is found to be out of adjustment, it may be corrected byadjusting rheostat R2. (See CAUTION at beginning of INSTALLATION PROCEDURESsection. )
THROUGH-CURRENT RESTRAINT
The through-current restraint, which gives the relay the percentage differential orpercent slope characteristics, is shown in Figure 7. It may be checked and adjustedusing the circuit illustrated in Figure 15, with S2 closed to position 13. AmmeterI reads the differential current and 13 reads the smaller of the two through-currents.
[ CAUTIONThese currents should be permitted to flow for only a few seconds at atime, with cooling periods between tests; otherwise, the coils will beoverheated -
NOTE: The percent slope tolerance is 10% of nominal, all in the plusdirection. This is to ensure that the slope characteristic never falls belowset point value.
In testing STD16C relays, the setting should be checked with switch 54 first in oneposition and then the other, thus checking all the restraint coils. With thecurrent tap plugs in 5.0 (1.0) ampere position and the percent slope set in the 40%position, the relay should just pick up for values of the 1i and 13 currentsindicated in Table V (VA). Repeat with the percent slope tap set in the 25% and 15%positions. If any one of these set points is found to be other than as prescribed,the adjustment may be made made by adjusting R3. It should be noted that thecurrent magnitude in the through-current branch (13) is slightly influenced by theapplication of differential current (Ii) and should be checked to make sure that itis maintained at its proper value.
TABLE V TABLE VA
%SLOPE AMPERES TRUE SLOPE JSETTING 13 (11/13 X 100)
40 30 12.0-13.2 40.0—44.025 30 7 .5- 8.3 25.0—27.515 30 4.5- 5.0 15.0—16.5
% SLOPE AMPERES TRUE SLOPESETTING 13 Ii (1/3 X 100)
40 6 2 .4-2 .64 40-4425 6 1 .5-1 .66 25-27.5
L 15 6 0 .9—1 .00 15—16.5
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GCK—45307
Any change in R3 to give the desired slope will have small effect upon minimumpickup and harmonic restraint. However, after the slope setting has once been set,any adjustment of minimum pickup will change the slope characteristics. The slopeset points must then be rechecked, to make sure that they are still in accordancewith Table III.
INSTANTANEOUS OVERCURRENT UNIT
This unit is located at the upper right-hand side of the relay. Its setting may hechecked by passing a high current of rated frequency through terminals 5 and 6, Theunit should pick up at 8 times the tap rating, as described under CHARACTERISTICS.If the setting is incorrect, it may be adjusted by loosening the lock nut at the topof the unit and turning the cap screw until the proper pickup is obtained. Inmaking this adjustment, the current should not be allowed to flow for more thanapproximate’y one second at a time.
DROPOUT OF MAIN UNIT
After the other tests are complete, check the dropout of the main unit, as describedin the ACCEPTANCE TESTS section.
LOCATI ON
The location should be clean and dry, free from dust and excessive vibration, andwell lighted to facilitate inspection and testing.
MOUNTING
The relay should be mounted on a vertical surface. The outline and panel drillingdimensions are shown in Figure 18.
CONNECTI ONS
Internal connection diagrams are shown in Figures 10 and 11. Typical wiringdiagrams are given in Figures 1, 2 and 3 for differential applications.
Of course, any through-current transformer winding may be used for any powertransformer winding, provided the taps are properly chosen.
When the relay is mounted on an insulating panel, one of the steel supporting studsshould be permanently grounded by a conductor not less than No. 12 AWG gage copperwire, or its equivalent.
Every circuit in the drawout case has an auxiliary brush. (See Figure 12.) This isthe shorter brush in the case, which the connecting plug should engage first. Onevery current circuit or other circuit with shorting bars, make sure the auxiliarybrushes are bent high enough to engage the connecting plug or test plug before themain brushes; otherwise the CT secondary circuit may be opened where one brushtouches the shorting bar before the circuit is completed from the plug to the other,main brush.
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GEK-45307
ADJUSTMENTS
TAP PLUG POSITIONING
Ratio Matching Adjustment
To obtain a minimum unbalance current in the differential circuit, means areprovided in the STD relay to compensate for unavoidable difference in current-transformer ratios. Taps on the relay transformer primary windings are rated 8.7(1.74), 5.0 (1.0), 4.6 (0.92), 3.8 (0.76), 3.5 (0.7), 3.2 (0.64), and 2.9 (0.58)amperes for each line—current transformer. The tap plugs should be moved to thelocations which most nearly match the expected CT currents for the same kVA assumedIn each of the power transformer windings. The selection of taps should be guidedby the method outlined under CALCULATIONS.
CAUTIONThe connection plug must be removed from the relay before changing tappositions, in order to prevent open-circuiting a CT secondary. A checkshould be made after changing taps, to ensure that only one plug Is leftin any horizontal row of tap holes. Inaccurate calibration andoverheating may result If more than one plug is connected to any onewinding.
Unbalance Current Measurement
Unbalance current measurement is useful in checking the best tap setting whenmatching current-transformer ratios in the field. It is also useful in detectingerrors or faults in the current transformer winding, or small faults within thepower transformer itself, where the fault current is too low to operate the relay.
The type STD relays have a special arrangement for measuring the unbalance currentflowing in the differential circuit without disturbing the relay connections.Provision is made for temporarily connecting a 5 V high—resistance AC voltmeter(1000 or more ohms per volt) across the secondary of the differential currenttransformer. This may be done by connecting the meter across terminals 8 and 9 (seeFigures 10 and 11). When a perfect match of relay currents is obtained by theratio-matching taps, the voltmeter will read 0, indicating no unbalance. If thevoltmeter reads 1.5 volts or less, the unbalance current entering or leaving a giventap equals approximately 0.03 times the voltmeter reading times the tap rating. Forhigher voltmeter readings, the approximate unbalance current may be calculated bysubstituting the voltage reading and tap rating into the following equation:
I (unbalance) = (0.16 x V — 0.2) x Tap
The unbalance percentage equals 100 times the unbalance current divided by the
measured tap current. For a three—winding bank, this must be checked with load onat least two pairs of windings in order to ensure that the connections are correct.
The curves in Figure 17 show the approximate voltage across terminals 8 and 9required to operate the relay, for various percent slope tap settings, and through—currents expressed as percentage of tap. To ensure a margin of safety against falseoperation, the unbalance voltage should not exceed 75% of that required to operatethe relay for any given through-current and percent slope tap setting. This extentof unbalance may result from the relatively high error currents of low-ratio bushingCTs at low multiples of tap current.
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GEK-45 307
This curve represents the STD relay characteristic. Measurement of a voltage across
StLLdS 8 and 9 which is 75% or less of the value given on the curve, does not
necessarily indicate that the relay will operate at higher through-current values.
This is especially true where very high through-faults may cause saturation.
Small rectifier-type AC voltmeters are suitable for the measurement of unbalance.
The voltmeter should not be left permanently connected, since the shunt current it
draws reduces the relay sensitivity.
PERCENT SLOPE SETTING
Scribe marks for 15, 25 and 40% slope settings are provided in both the STD15 and
STD16 relays. It is common practice to use the 25% setting unless special
connections make it advisable to use one of the others. See the correspondingheading under CALCULATIONS for further details.
OPERATI ON
TARGETS
Targets are provided for both the seal-in unit and the instantaneous overcurrentunit. In the event of an internal fault, one or both of these units will operate,
depending upon the fault magnitude. This will produce a target indication of theparticular unit which operates. After a fault is cleared, the target should bereset by the reset slide located at the lower left—hand corner of the relay.
DISABLING OF TYPE STD RELAY
CAUTION
When bypassing a breaker for maintenance, it will be necessary to disablethe relay to prevent false tripping.
If the disabling of the relay is done by a remote switch, rather than by removingthe relay connection plug, the following precautions should be taken:
1. The relay must be disabled by short—circuiting studs 8 and 9 of the relay, orby opening the trip circuit at stud 1.
2. If the CT secondaries are short-circuited as part of the disabling procedure,the trip circuit should be opened at stud 1, and studs 8 and 9 should be short-circuited first. It is not sufficient to rely on short circuiting the CTsecondaries alone, because any difference in time of shorting them may causefalse tripping.
MAI NTENAfCE
CONTACT CLEANING
For cleaning fine silver contacts, a flexible burnishing tool should be used. Thisconsists of a flexible strip of metal with an etched-roughened surface, resembling
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GEK—45 307
in effect a superfine file. The polishing action is so delicate that no scratchesare left, yet corroded material will be removed rapidly and thoroughly. Theflexibility of the tool insures the cleaning of the actual points of contact.
Fine silver contacts should not he cleaned with knives, files, or abrasive paper orcloth. Knives or files may leave scratches, which increase arcing and deteriorationof the contacts. Abrasive paper or cloth may leave minute particles of insulatingabrasive material in the contacts, and thus prevent closing.
The burnishing tool described above can be obtained from the factory.
PERIODIC TESTS
An operation test and inspection of the relay and its connections should be made atleast once every six months. Tests may be performed as described under INSTALLATIONTESTS, or if desired they may be made on the service taps as described in thissection.
When inserting or withdrawing a test plug with U—shaped through jumpers to completethe trip circuit through the test plug, similar through jumpers should also be usedon studs 8 and 9 to maintain the connections from the relay to the case. If this isnot done, there is a risk of false tripping upon inserting or withdrawing the plug.
PICKUP
The method for checking pickup is as described under the heading INSTALLATION TESTSexcept, of course, pickup current will be different depending upon the winding (WDG)1 service tap. Pickup may be determined as follows:
I 0.30 x WDG 1 Tap
Of course, when checking pickup on a particular service tap, the 10% expectedvariation still applies, the acceptable-as—found range being
Ii 9.0 x 0.30 x WDG 1 Tap to 1.10 x 0.30 x WDG 1 Tap
Example WDG1 Tap = 3.5AIi = 0.90 x 0.30 x 3.5 to 1.10 x 0.30 x 3.5
= 0.94 to 1.16 amperes
HARMONIC CURRENT RESTRAINT
The procedure for checking harmonic restraint is as described under the headingINSTALLATION TESTS, except that the test current values must be modified as follows:
12 (DC) = 0.80 x WDG 1 TapIi = 0.90 x WDG 1 Tap to 1.10 x WDG 1 Tap
In the event a suitable DC meter is not available, 12 (AC) = 2.25 x I? (DC).(Theoretically, this conversion factor would be 2.22 if the rectifier backresistance were infinite.)
GEK—45307
Example WDG 1 Tap = 3.SA12 (DC) 0.80 x 3.5 = 2.8 amperes
I = 0.90 x 3.5 to 1.10 x 3.5Ii = 3.15 to 3.85 amperes
If DC meter is not available, 12 (AC) would in this example be2.25 x 2.8 = 6.30 amperes.
THROUGH-CURRENT RESTRAINT
In order to check the service tap slope setting, the test current values indicatedin Table IV must be modified to take into account any difference in tap settings.Furthermore, the test circuit shown in Figure 15 must be set up such that the leadfrom ammeter 13 to the test plug is connected to the stud that connects to thewinding with the lowest tap setting. The common lead, of course, is connected tothe stud corresponding to the winding with the higher tap setting. For anycombination of taps, the percent slope is given in the following equation.
% Slope
= [ (+ ) -i x 100
where= smaller tap setting
T2 = higher tap settingdifferential current
13 = smaller of the two through currents
Table VI, derived from the above expression, is based on a multiple of tap currentof 6 times the lower tap setting for all combinations of taps except those whichinvolve the 8.7 amp tap. For the latter case, a 4—times tap setting is used, sincethe total test current for a 6-times tap setting may be as high as 75.2 amperes,which is not only prohibitively high for many installations, but also may subjectthe relay to excessive heating.
For a given tabular value of 13 corresponding to a given combination of winding andpercent slope taps, the values of 1 (iniri.) and i (max.) correspond to the minimumand maximum percent slope tolerance limits given in Table V. However, for a 4-times tap setting, both the upper and the lower percent tolerance limits have beenraised by a value equivalent to the difference between the true slope and thenominal slope at 4-times tap value, indicated by the percent slope characteristiccurves shown in Figure 7.
Example WDG 1 Tap = 3.5 AWdg 2 Tap 5.0 ASlope Setting = 40%
Since WDG1 has the lower tap setting, the lead from ammeter 13 to the test plugshould be connected to stud 6, and the common lead connected to stud 4.
From Table VI
13 = 21.0 amps1i (mm.) = 21.0 amps11 (max.) = 22.2 amps
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GEK—45307
SERVICING
Check any replacement telephone relay for mechanical operation before Installation,to see that it operates smoothly and that the contacts are correctly adjusted.
With the relay de—energized, each normally—open contact should have a gap of 0.010
0.015 Inch. Observe the wipe on each normally-closed contact by deflecting the
stationary contact member towards the frame. Wipe should be approximately 0.005
inch.
The wipe on each normally-open contact should be approximately 0.005 Inch. Check by
inserting a 0.005 inch shim between the residual screw and the pole piece, and
operating the armature by hand. The normally—open contacts should make before the
residual screw strikes the shim.
RENEWAL PARTS
Sufficient quantities of renewal parts should be kept in stock for the prompt
replacement of any that are worn, broken or damaged.
When ordering renewal parts, address the nearest Sales Office of the General
Electric Company. Specify the name of the part wanted, quantity required, and
complete nameplate data, including the serial number, of the relay for which the
part is required. The serial number may be found stamped on the instantaneous unit
in black ink. If possible, give the General Electric Company requisition number on
which the relay was furnished.
Since the last edition, the equation has been cahnged in the section on UnbalanceCurrent Measurement and Figure 7b has been added.
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GEK—43C7
TABLE VI
TAPS T2 2.9 3.2 3.5 3.8 4.2 4.6 5.0 8.7Slopes 15 2$ 41) 15 25 40 15 25 40 15 25 40 35 25 40 15 25 40 15 25 40 15 25 40
Ti Currents13 17.4 17.4 17.4 17.4 17.4 17.4 17.4 17.4 17.4 17.4 17.4 17.417.417.4 17.4 17.4 17.4 17.4 17.4 17.4 17.4 11.6 11.6 11.6
2.9 Ij(min) 2.6 4.3 7.0 4.6 6.6 9.5 6.7 8.8 12.0 8.8 11.1 14.5 11.6 14.1 17.9 14.3 17.1 21.2 17.1 20.1 24.6 28.7 32.2 37.4I1(max) 2.9 4.8 7.7 5.0 7.1 10.3 7.1 9.4 12.8 9.2 11.7 15.4 12.0 14.8 18.9 14.8 17.8 22.4 17.6 20.9 25.8 29.3 33.1 38.9Ij 19.2 19.2 19.2 19.2 111.219.219.2 19.2 10.2 19.2 19.2 19.2 19.2 19.2 19.2 19.2 15.2 19.2 12.8 12.8 12.8
3.2 I(min) 2.9 4.8 7.7 4.9 7.0 10.2 7.0 9.3 12.7 9.8 12.3 16.1 12.5 15.3 19.4 15.3 18.3 22.8 27.5 31.0 36.21i(ulax) 3.2 5.3 8.5 5.3 7.6 11.0 7.4 9.9 13.6 10.2 13.0 17.1 13.0 16.0 20.6 15.8 19.1 24.0 28.1 31.9 37.713 - 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 14.0 14.0 14.0
3.5 11(inin) 3.1 5.2 8.4 5.2 7.5 10.9 8.0 10.5 14.3 10.7 13.5 17.6 13.5 16.5 21.0 26.3 29.8 35.0Ii(max) 3.5 5.8 9.3 5.6 8.1 11.8 8.4 11.2 15.3 11.2 14.2 18.8 14.0 17.3 22.2 26.9 30.7 36.5J3 22.8 22.8 22.8 22.8 22.82.2.8 22.8 22.8 22.8 22.82.2.8 22.8 15.2 15.2 15.2
3.8 11(niin) 3.4 5.7 9.1 6.2 8.7 12.5 8.9 11.7 15.8 11.7 14.7 19.2 25.1 28.6 33.8Ii(max) 3.0 6.3 10.0 6.6 13.5 9.4 12.4 17.0 12.2 15.5 20.4 25.7 29.5 35.313 25.2 25.2 25.2 25.2 25.2 25.2 25.2 25.2 25.2 16.8 16.8 16.8
4.2 lj(min) 3.8 6.3 10.1 6.5 9.3 13.4 9.3 12.3 16.8 23.5 27.0 32.2Ii(max) 4.2 7.0 11.1 7.0 10.0 14.6 9.8 13.1 18.0 24.1 27.9 33.7
3 — 27.6 27.6 27.6 27.6 27.6 27.6 18.4 18.4 18.44.6 i(min) 4.1 6.9 11.0 6.9 9.9 14.4 21.9 25.4 30.6
](max) 4.6 7.6 12.2 7.4 10.7 15.6 22.5 26.3 32.130.0 30.0 30.0 20.0 20.0 20.0
5.0 11(min) 4.5 7.5 12.0 20.3 23.8 29.011(max) 5.0 8.3 13.2 20.9 24.7 30.5
13 34.8 34.8 34.88.7 11(min) 5.5 9.0 14.2
11(max) 6.1 9.9 15.7
TABLE VIA
I1Ai’S T2 0.58 0.64 0.7 0.76 0.4 0.92 1.0 1.74Slopes 15 25 40 15 25 40 15 25 40 15 25 40 15 25 40 15 25 40 15 25 40 15 25 40
T2 Currents13 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 2.3 2.3 2.3
0.68 11(min) 0.52 0.86 1.4 0.92 1.32 1.9 1.34 1.76 2.4 1.76 2.22 2.9 2.32 2.82 3.6 2.86 3.42 4.24 3.42 4 4.92 574 6.44 7.41j(max) 0.58 0.96 1.54 1.0 1.42 2.06 1.42 1.88 2.56 1.84 2.34 3.08 2.4 2.96 3.78 2.96 3.56 4.5 3.52 4.18 5.16 5.86 6.62 7.7
13 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.83.8 2.56 2.56 2.50.64 li(min) 0.58 0.96 1.54 0.98 1.4 2 1.4 1.86 2.54 1.96 2.46 3.22 2.5 3.06 3.88 3.06 3.66 4.56 3.5 6.2 7.2
Ij(max) 0.64 1.06 1.7 1.06 1.52 2.2 1.48 1.98 2.72 2.04 2.6 3.42 2.6 3.2 4.12 3.16 3.82 4.8 5.62 6.38 7.5
13 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 2.8 2.8 2.80.7 J1(mn) 0.62 1.04 1.63 1.04 1.5 2.18 1.6 2.1 2.86 2.14 2.7 3.52 2.7 3.3 4.2 5.26 5.96 713(max) 0.7 1.16 1.86 1.12 1.62 2.36 1.68 2.24 3.06 2.24 2.84 3.76 2.8 3.46 4.44 5.38 6.14 7.313 4.5 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 3.0 3.0 3.0
0.76 11(rnin) 0.68 1.14 1.82 1.24 1.74 2.5 1.78 2.34 3.16 2.34 2.94 3.84 5.0 5.72 6.7l(max) 0.76 1.26 2.0 1.32 1.88 2.7 1.88 2.48 3.4 2.44 3.1 4.08 5.14 5.9 7.013 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 3.36 3.36 3.3
0.84 11(min) 0.76 1.26 2.0 1.3 1.86 2.68 1.86 2.46 3.36 4.7 5.4 6.4I(max) 0.84 1.4 2.22 1.4 2.0 2.92 1.96 2.62 3.6 4.82 5.58 6.7
; 13 5.5 5.5 5.5 5.5 5.5 5.5 3.7 3.7 3.70.92 11(min) 0.82 1.38 2.2 1.38 1.98 2.88 4.38 5.1 6.1
1 (max) 0.92 1 .52 2.44 1.48 2.14 3.02 4.5 5.26 6.4
13 6.0 6.0 6.0 4.0 4.0 41.0 I1(min) 0.90 1.5 2.4 4.06 4.76 5.8
Ii(max) 1 1.66 2.64 4.18 4.94 6.113 7.0 7.0 7.0
1.14 J1(min) 1.1 1.8 2.8
[ I(max) 1.22 1.98 3.1
27
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32
NOTE: FOR TWO WINDING TRANSFORMER RELAYS‘THROUGH CURRENT” IS TAKEN AS THE SMALLEROF THE TWO CURRENTS. FOR THREE WINDINGTRANSFORMERS, IT IS TAKEN AS THE SUM OFTHE INCOMING OR OUTGOING CURRENTS, WHICHEVER IS SMALLER. (EACH CURRENT TO BEEXPRESSED AS A MULTIPLE OF TAP.)
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GE Power Management
215 Anderson AvenueMarkham, OntarioCanada L6E 1B3Tel: (905) 294-6222Fax: (905) 201-2098www.ge.comli ndsyslpm
